Historically Americans came from Old English, but beyond that, Americans usually vary more because most are bred for fighting, not show, thus you have different named lines and most of them are squirrel tailed or with tails carried somewhere between 50-90 degrees above horizontal.

American Games are found with a much closer dubbing in males, beyond that it is a bit hard to say any true differences because they do vary. There are different leg colors, comb types, body shapes/carriage, tail expansions, etc.

So in a nutshell, American Games are basically a large group of birds very similar and originating from Old English, but many Americans are still bred more for fighting purposes than show. With that in mind, they vary, especially according to their lineage.

90 degrees or more. It's a disqualification for any breed in showing, but common in American Games.

The close dubbing is for fighting, yes, but I believe (don't really know though) that Old English are dubbed less harshly for both protection from frostbite to the points and also for a more appeasing look. I don't really know the reasons for Old English dubbing style though, I just know that it shouldn't be too close the the skull, and usually just lops off the points and makes for a finely drawn curve on the comb. American Games are most often down almost to the skull, where you just don't even realize they ever had a comb.